Fix regression since 2a50b8b6bb (#597).
Dimension is intended to be embedded in various clients,
be it the Element service that we host (at element.DOMAIN),
some other Element (element-desktop running locally), etc.
The when statement is supposed to be on the block, not on the individual task.
It affects all tasks within the block (they're all to be executed when ma1sd is enabled and self-building is requested0.
The tag format used in the `ma1sd` repo have change. Versions no longer
start with 'v', and when building for non-amd64, we also need to strip
off the '-$arch' bit from the Docker image name.
Further, when building the .jar file, `ma1sd` currently names the .jar
based on the project's directory, which we call 'docker-src'. This means
other parts of the `ma1sd` build can't find the .jar file. Remedy this
by ensuring that the dir is called `docker-src/ma1sd`.
`matrix_container_images_self_build` was not really doing anything
anymore. It previously was influencing `matrix_*_self_build` variables,
but it's no longer the case since some time ago.
Individual `matrix_*_self_build` variables are still available.
People that would like to toggle self-building for a specific component
ought to use those.
These variables are also controlled automatically (via
`group_vars/matrix_servers`) depending on `matrix_architecture`.
In other words, self-building is being done automatically for
all components when they don't have a prebuilt image for the specified
architecture. Some components only support `amd64`, while others also
have images for other architectures.